Even before last week's terrorist attacks in Paris, the French prime minister was concerned about the continued viability of Jewish life in France. In an interview with The Atlantic prior to the Charlie Hebdo and kosher supermarket massacres, Manuel Valls made a grim prediction:

"If 100,000 Jews leave, France will no longer be France. The French Republic will be judged a failure."

His misgivings were far from groundless. An exodus of French Jews is already underway and accelerating rapidly. In 2012, there were just over 1,900 immigrants to Israel from France. The following year nearly 3,400 French Jews emigrated; in 2014 approximately 7,000 left. For the first time ever, France heads the list of countries of origin for immigrants to Israel, and the ministry of immigration absorption expects another 10,000 French Jews to arrive in 2015.

That would mean more than 22,000 Jews fleeing France for Israel in the space of just four years, nearly 4.5 percent of the country's Jewish population. The departure of 100,000 French Jews might once have been inconceivable. No longer. In a survey last spring of France's Jewish community, the largest in Europe, three out of four respondents said they were considering emigrating.

These are staggering numbers — all the more so in a "Jewish community that has been in place for centuries and feels itself deeply attached to being French," as Daniel Jonah Goldhagen has written. But what is driving so many Jews to leave "is not Israel's pull…. It is France's push."

To his credit, the French prime minister has been forthright in his condemnation of Islamist anti-Semitism. Over the weekend he declared that France was at war "against terrorism and radical Islam," a war in which "journalists were killed for drawing" and "Jews were killed because they were Jewish." But strong words from a prime minister will not halt the antisemitic derangement, and Valls is right to fear what that derangement would mean to France's future well-being.

Anti-Semitism is commonly regarded as a variety of racism, but the prolific English historian Paul Johnson suggests that it should be seen as a kind of intellectual disease, fundamentally irrational and highly infectious. It exerts greatself-destructive force, Johnson wrote in a notable 2005 essay, severely harming countries and societies that engage in it. In a pattern that has recurred so predictably that he dubbed it a "historical law," nations that make Jewish life untenable condemn themselves to decline and weakness.

For example, Spain's expulsion of the Jews in the 1490s, and its subsequent witchhunt of the converted "New Christians" who remained behind, meant a loss of Spanish financial and managerial talent at the very moment the New World was being opened up to lucrative colonization. That had "a profoundly deleterious impact," Johnson argued, "plunging the hitherto vigorous Spanish economy into inflation and long-term decline, and the government into repeated bankruptcy." More than 500 years later, Spain — where, incidentally, Valls was born and lived until his teens — still regrets that self-inflicted wound, and has looked for ways to rectify it.

Johnson pointed out other prominent examples of the phenomenon. Czarist Russia's persecution of Jews, reinforced by the encouragement of brutal pogroms, fueled a massive migration of Jews to the West, especially to Britain and the United States; those countries' cultural and entrepreneurial gain was Russia's debilitating loss. Germany's descent into demonic Jew-hatred under the Nazis ended in devastating military defeat, followed by a decades-long Cold War rupture and the end of German renown as Europe's intellectual center. The Arab world, steeped in antisemitism and obsessed with the Jewish state, squandered vast oil riches "on weapons of war and propaganda," wrote Johnson. "In their flight from reason, they have failed to modernize or civilize their societies, to introduce democracy, or to consolidate the rule of law." Arab culture once led the world in learning, innovation, and pluralism. Today it is a world leader in almost nothing, save fratricidal violence and Islamist fanaticism.

France's Jews are leaving, and that never bodes well for the society driving them out. The prime minister puts his finger on it: If there is no Jewish future in France, if the antisemitic cancer has metastasized so alarmingly that tens of thousands of French Jews are ready to flee, then France will indeed no longer be France. It will be something darker and more deformed, wrecked by an injury it inflicted on itself.